Vaccines Could Help Reduce Asymptomatic Virus Transmission – Although More Evidence Is Needed
Numbers suggest that the Moderna vaccine reduced asymptomatic transmission by about two-thirds. The data set was very small, however.

Moderna’s mRNA COVID-19 vaccine appears to be able to reduce infection and therefore transmission of the virus, a Harvard expert has told Health Policy Watch, albeit despite the currently limited data sets.

Although both vaccines appear to have incredibly high efficacy (94.1% for Moderna and 95% for Pfizer) in terms of preventing those vaccinated from becoming ill with COVID-19, policymakers have cautioned that people thus immunized might still be able to transmit the virus to others – and public health policies need to take this into account.

Peer reviewed Phase 3 clinical trial results for the Moderna vaccine, published last week in the New England Journal of Medicine, now suggest, however, that vaccination also reduced asymptomatic transmission by about two-thirds, in the case of Moderna’s mRNA-1273 vaccine – although the numbers examined were small.

Pfizer/BioNTech, however, said that their Phase 3 data did “not address whether vaccination prevents asymptomatic infection” and that “a serologic end point that can detect a history of infection regardless of whether symptoms were present … will be reported later.”

“It would be safe to conclude from the very limited findings that the mRNA vaccine has some effect on infection and therefore probably transmission, but it’s a pretty small data set to hang your hat on“, Marc Lipsitch, Professor of Epidemiology and Director of the Center for Communicable Disease Dynamics at the Harvard T.H. Chan School of Public Health, told Health Policy Watch during a media advisory on Tuesday.

Pfizer, which did not address whether vaccination prevents asymptomatic infection in this trial, will report the data when it is available.

In the Moderna study, there were just 15 asymptomatic cases of infection in the mRNA-1273 group of about 14,000 who received the vaccine, or about 0.1% of participants at the time that the second vaccine dose was administered. In comparison there there were 39 symptomatic cases in the  placebo group of a comparable size, or about 0.3% of participants – suggesting that even one dose of the two-dose vaccine had already reduced asymptomatic cases by roughly one-third.

That is extremely encouraging to policymakers looking at how vaccination of large numbers of people can also halt the broader spread of infection.

But this data remains “hard to interpret, for a number of reasons”, Lipsitch said, noting that: “People getting their second dose could have been infected before the first dose kicked in. Moreover, the second dose probably adds more protection compared to the first dose.

He added that in addition, “we don’t know the duration of that effect, or the degree to which the vaccine changes the amount of virus in [people].”

Regardless of Vaccination – Continue Testing

Regardless, Lipsitch still advised governments to continue testing requirements for traveling, as opposed to banking entirely on vaccination for future out-of-country movement. “It’s not completely protective so testing would be more meaningful than the vaccine,” he said.

Though more research is needed to confirm how long immunity from SARS-CoV-2 lasts, Lipsitch estimated that the immunity lasts at least three months, depending both on the product and on individual contribution and responsibility to be protected for longer.

With new variants of SARS-CoV-2 emerging in the United Kingdom and South Africa, there must be redoubled efforts to both control the spread of COVID-19 and to vaccinate as many people as possible. “Anything you can do to delay the spread of this new variant virus will make control easier and will help us in a race to get more people vaccinated before [this variant] becomes more common.”

This may be a challenge in coming weeks, with the new variant 50-70% more transmissible, but ongoing vaccinations and further research gives hope that herd immunity can be reached.

Said Eric J. Rubin and Dan L. Longo of the NEJM, regarding the new vaccines: “What appears to be a dramatic success for vaccination holds the promise of saving uncounted lives and giving us a pathway out of what has been a global disaster.”

Image Credits: Moderna, Pfizer.

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